Method and apparatus for tuning an nmr field coil

Abstract

A method of tuning a radio frequency coil for use in MR imaging applications uses reactive shunts having a reactance in parallel with a switch to connect sections of the coil to ground thereby constraining the resonant mode of the coil to a known orientation. A plurality of measurements of the frequency of the resonances along different such constrained orientations is used to deduce the frequencies of the two unconstrained orthogonal resonant modes of the coil and its orientation. The shunts may also be used to tune the coil according to the deduced frequencies of the unconstrained resonance and the desired frequency and orientation by adjusting the reactance of the shunts. A shunt having a variable reactance and a switch for shorting the RF coil as required uses a PIN diode in combination with a variable reactance.

I claim: 
     
       1. A method of determining the frequencies f 1  and f 2  and orientations θ 1  and θ 2  of orthogonal resonant modes of an unconstrained resonant NMR RF coil comprising the steps of: a) constraining the orientation of a resonant mode of the RF coil to a given θ;   b) measuring the frequency of the constrained resonant mode;   c) repeating steps (a) and (b) to obtain a plurality of different frequency measurements associated with different orientations θ;   d) deriving the frequencies f 1  and f 2  and orientations θ 1  and θ 2  of the orthogonal resonant modes of the unconstrained RF coil from the plurality of different frequencies measurements.   
     
     
       2. The method of claim 1 wherein the plurality of different frequency measurements comprise four frequency measurements f a , f b , f c , and f d  where f a  is constrained to an arbitrarily chosen orientation of 0° and where f b , f c , and f d  are taken at orientations displaced successively by 45° from that of f a  and where the frequencies f 1  and f 2  and orientations θ 1  and θ 2  are calculated from the following equations: ##EQU16## 
     
     
       3. The method of claim 1 wherein the orientation of the resonant mode in the RF coil is constrained to a given θ by the use of shunts attached to the coil at predetermined points for connecting those points to ground. 
     
     
       4. The method of claim 1 wherein the orientation of the resonant mode in the RF coil is constrained to a given θ by the use of switches attached to the coil at predetermined points opening the coil at those points. 
     
     
       5. A method of tuning an unconstrained NMR resonant RF coil to a desired resonant mode, the coil having variable reactance shunt pairs at angles 0°, 45°, 90° and 135° each shunt with variable impedances z(φ=0°), z(φ=45°), z(φ=90°), and z(φ=180°) respectively, and having an impedance sensitivity of k b  and having untuned orthogonal resonant modes with frequencies f 1  and f 2  respectively and orientation θ 1  comprising the steps of: determining a difference in frequency Δf 1  and Δf 2  between the desired resonant frequency and f l  and f 2  respectively;   calculating the changes of impedances of the shunts required to tune the RF coil according to the following equations: ##EQU17##